Photocapacitance Measurements Research Articles

Plasma-Induced Damage and Recovery on Au/n-GaN Schottky Diode in Different Processes

The effects of plasma-induced damage on deep traps in n-GaN have been investigated using current–voltage (I–V), capacitance–voltage (C–V), and photocapacitance (PHCAP) measurements. The Au/n-GaN Schottky barrier diodes were fabricated in an inductively coupled plasma ion etching (ICP-RIE) system. After mesa etching to achieve ohmic contact, the n-GaN surface, at which Schottky contacts are fabricated, is etched ∼100 nm by ICP-RIE with various Cl2/Ar ratios and RIE bias powers (PB), to introduce plasma damage. The electrical properties of the fabricated Shottky barrier diodes (SBDs) strongly dependent on the RIE gas composition and the bias power PB applied to the sample stage. In order to overcome the residue and plasma damage on the Schottky area, the samples were treated with HCl at 110 °C for 30 min. Several deep levels (1.8, 2.5, and 3.0 eV below the conduction band) were detected by PHCAP measurement. Improved electrical characteristics were achieved as a result of the HCl treatment and sintering process. The PHCAP measurement results also revealed the effectiveness of thermal and chemical treatments.

Effects of illumination on capacitance characteristics of Au/3C-SiC/ p-Si/Al diode

Au/3C-SiC/ p-Si/Al Schottky barrier diode was prepared using atmospheric pressure chemical vapor deposition technique. The device parameters such as barrier height, ideality factor, and series resistance were calculated using current–voltage characteristics, and were found to be 0.44 eV, 1.55, and 1.02 × 10 4 Ω, respectively. The photocapacitive properties of the diode were studied under various illumination intensities. The transient photocapacitance measurements indicate that the capacitance of the Au/3C-SiC/ p-Si/Al Schottky diode is very sensitive to illumination. The photocapacitance of the diode increases with increase in illumination intensity. The increase in photocapacitance with increase in illumination intensity suggests that these devices could be utilized as a photocapacitive sensor for optical sensors.

Dynamics of Bulk Polymer Heterostructure/Electrolyte Devices

The application of organic semiconductors and bulk heterojunction (BHJ) devices in photoelectrochemical cells, electrolyte-gated field effect transistors, and neuromorphic devices involves the interface of the polymer with an electrolyte. We report the observation of interesting features arising from bulk and interfacial properties of stable polymer/electrolyte devices from photovoltage and differential photocapacitance measurements. A crossover in polarity of the photovoltage signal as a function of BHJ layer thickness and a crossover in the sign of differential photocapacitance as a function of frequency are observed in certain classes of these device structures. The presence of the critical thickness and crossover frequency can be understood in the framework of an electrical transport model and the parameters defining the interfacial capacitance.

Deep Electronic Levels of AlxGa1-xN with a Wide Range of Al Composition Grown by Metal–Organic Vapor Phase Epitaxy

Deep electronic levels of AlxGa1-xN (0.25<x<0.60) were investigated by deep level transient spectroscopy (DLTS) and photocapacitance methods. Si-doped AlGaN layers were grown on an AlN/sapphire template by metal–organic vapor phase epitaxy. DLTS analysis using a sampling time window of up to 100 s showed two dominant deep levels with activation energies (ΔE) higher than 1.0 eV in AlxGa1-xN with x=0.25 and 0.37. The densities of those levels were higher than 1×1016 cm-3. For the Al0.60Ga0.40N sample, the deeper levels (ΔE>1.5 eV) were detected by photocapacitance measurement. It was found that the energy position of the dominant deep level closely followed the Fermi level stabilization energy reported by Walukiewicz et al. [J. Cryst. Growth 269 (2004) 119], indicating that the origin of the dominant deep level in AlGaN is related to a defect complex including anti-site defects and divacancies.

ChemInform Abstract: Primary Reactions in the Photocorrosion of CdSe Through Photocapacitance Measurements.

CdSe/liquid junctions have been investigated through photocapacitance measurements in single and in double beam conditions

Photoionization study of deep centers in GaN∕AlGaN multiple quantum wells

Transient photocapacitance (TPC) measurements were performed to investigate deep centers in GaN∕AlGaN multiple quantum wells. The influence of the persistent photovoltaic effect was successfully separated during the TPC experiments. The resolution obtained by the TPC measurements is much better than that of steady-state photocapacitance. The spectral dependence of photoionization cross section of deep centers in GaN is quantitatively determined in the energy range from 1.68to3.30eV. The absolute values of photoionization cross sections of these centers are found to be of the order of 10−15–10−14cm2.

Reduction of Low-Temperature Nonlinearities in Pseudomorphic AlGaAs/InGaAs HEMTs Due to Si-Related DX Centers

The linearity of conventional pseudomorphic AlGaAs/InGaAs/AlGaAs high-electron mobility transistors with planar doping in the AlGaAs layers is shown to degrade at low temperatures down to -40°C, as measured by the adjacent-channel power ratio under wideband code-division multiple-access modulation. A modified structure, in which the planar Si doping layers are placed within thin single GaAs quantum wells inside the AlGaAs barrier layers, eliminates this degradation. Deep-level transient spectroscopy and persistent photocapacitance measurements show that trapping on DX centers is effectively eliminated. The linearity improvements are therefore attributed to the elimination of this trapping. Self-consistent solutions of the Schro?dinger and Poisson equations show that the transfer of the donor electrons into the channel is essentially the same in the modified and conventional structures.

Junction Capacitance Study of a-SiGe:H Solar Cells Grown at Varying RF and VHF Deposition Rates

AbstractSignificant advances have been made in increasing the deposition rate of hydrogenated silicon germanium alloys (a-SiGe:H) using a modified VHF glow discharge deposition method while also maintaining good electronic properties important for its application in photovoltaic devices. We examine the electrical and optical properties of these alloys deposited either by RF (13.56MHz) or the modified VHF methods over deposition rates varying from 1 to 10 Å/s. The electronic properties of a series of 1.4 eV optical gap a-SiGe:H i-layers, in many cases in solar cell device configurations, were characterized. Drive-level capacitance profiling was used to determine the deep defect densities, and transient photocapacitance measurements allowed us to determine the Urbach energies. Results were obtained for both the annealed and light-soaked degraded states and these results were correlated to the cell performance parameters. In general the a-SiGe:H layers deposited using the modified VHF excitation exhibited improved electronic properties at higher growth rates than the RF deposited samples.

Characterization of plasma etching damage on p-type GaN using Schottky diodes

The plasma etching damage in p-type GaN has been characterized. From current-voltage and capacitance-voltage characteristics of Schottky diodes, it was revealed that inductively coupled plasma (ICP) etching causes an increase in series resistance of the Schottky diodes and compensation of acceptors in p-type GaN. We investigated deep levels near the valence band of p-type GaN using current deep level transient spectroscopy (DLTS), and no deep level originating from the ICP etching damage was observed. On the other hand, by capacitance DLTS measurements for n-type GaN, we observed an increase in concentration of a donor-type defect with an activation energy of 0.25eV after the ICP etching. The origin of this defect would be due to nitrogen vacancies. We also observed this defect by photocapacitance measurements for ICP-etched p-type GaN. For both n- and p-type GaN, we found that the low bias power ICP etching is effective to reduce the concentration of this defect introduced by the high bias power ICP etching.

Photocapacitance measurements in irradiated a-Si:H based detectors

Photocapacitance measurements were performed on amorphous silicon p–i–n detectors before and after particle irradiation with 1.5MeV 4 He+ ions. The spatial resolution across a degraded spot is similar to the one obtained in photocurrent scans and is of the order of the diameter of the scanning laser beam. We monitored the transient capacitance after applying short laser pulses to deduce trap energies of 0.64eV. Photocapacitance measurements as a function of the applied bias, the measurement frequency up to 1MHz, and the wavelength of laser light are discussed. The reduction in photocapacitance signal and the shift of the cut-off frequency after ion bombardment are correlated with the change in transport properties.

Spatially-resolved photocapacitance measurements to study defects in a-Si:H based p–i–n particle detectors

Thick large-area particle or X-ray detectors suffer degradation during operation due to creation of defects that act as deep traps. Measuring the photocurrent under homogeneously absorbed weak light can monitor variation in detector performance. We describe how photocapacitance can be used as an alternative method to measure the creation of defects and their energy level after intense irradiation with protons or He ions at 1.5 MeV and after exposure to intense laser pulses. The possibility to detect small areas of high defect density in a large-area detector structure is discussed.

Ultraviolet and visible photoresponse properties of n-ZnO∕p-Si heterojunction

A n-ZnO∕p-Si thin film heterojunction has been fabricated by a low cost sol-gel technique. The wavelength dependent photoresponse properties of the heterojunction is investigated in detail by studying the effect of light illumination on current-voltage (I-V) characteristics, photocurrent, and photocapacitance spectra at room temperature. It shows good diode characteristics with IF∕IR=3.4×103 at 4V and reverse leakage current density of 7.6×10−5Acm−2 at −5V. From the photocurrent spectra, it is observed that the visible photons are absorbed in the depleted p-Si under reverse bias conditions, while ultraviolet (UV) photons are absorbed in the depleted n-ZnO under positive bias conditions. This indicates that such a sol-gel n-ZnO∕p-Si thin film heterojunction can be used to sense both UV and visible photons though the photoresponse for UV is much slower than that of visible. The photocapacitance measurements suggest the presence of a shallow defect level in the sol-gel derived ZnO film which acts as an electron trap at ∼0.16eV below the conduction band.

Dislocation‐induced deep levels in ELO InP revealed by point contact photocapacitance measurements

30K-photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions. In the dislocation-free layers, the dominant deep level was located at Ec-1.30 eV, whereas in the dislocated area, dominant deep levels were detected at Ec-0.86 eV and 1.05 eV. A neutralized state was also detected at 0.66 eV+Ev. Excitation photocapacitance results have shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts of 0.28 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation-induced deep electronic states in InP: Photocapacitance measurements

Photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions by using wedge wire-bonding of Au, and photocapacitance measurements were then carried out at $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the dislocation-free layers, the dominant deep level was located at $1.30\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band, whereas in the dislocated area, dominant deep levels were detected at $0.86\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $(\ensuremath{\lambda}=1.44\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ and $1.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $(\ensuremath{\lambda}=1.18\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ below the conduction band. A neutralized state was also detected at $0.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band. From the detailed excitation photocapacitance results, it is shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts $({d}_{\mathrm{FC}})$ of $0.28\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This means that the atomic configurations around the deep levels are highly relaxed, as expected from the structures of the dislocation cores.

Electron trapping and inversion layer formation in photoexcited metal-insulator-poly(3-hexylthiophene) capacitors

Photocapacitance measurements are reported on metal-insulator-semiconductor (MIS) capacitors employing polyimide (PI) or polysilsesquioxane (PSQ) as the gate insulator and poly(3-hexylthiophene) as the active semiconductor. By stressing devices into depletion while simultaneously irradiating with light of energy exceeding the semiconductor band gap, photogenerated electrons become trapped at the insulator/semiconductor interface or possibly in bulk insulator states. Additionally for the PSQ device, evidence is provided for the formation of a photogenerated inversion layer at the interface. The time dependence of electron detrapping in the PI case is similar to that observed for accumulation stress instability in organic MIS devices.

Band offset measurements of ZnO∕6H-SiC heterostructure system

The conduction band offset of n-ZnO∕n-6H-SiC heterostructures fabricated by rf-sputtered ZnO on commercial n-type 6H-SiC substrates has been measured by a variety of methods. Temperature dependent current-voltage characteristic, photocapacitance, and deep level transient spectroscopy measurements showed the conduction band offsets to be 1.25, 1.1, and 1.22eV, respectively.

Investigation of deep level traps in dilute GaAsN layers grown by liquid phase epitaxy

Dilute GaAsN layers are grown by liquid phase epitaxy (LPE) technique, using polycrystalline GaN as the source of nitrogen. A band gap lowering of 100 meV is obtained from low temperature photoluminescence (PL) measurements which correspond to a little more than 0.5% nitrogen in the layer. Using 10 K photocapacitance measurements, a 0.7 eV electron trap is detected in the material which is assigned to an interstitial (N–N) As defect. Annealing of the material at 750 °C for 1 h greatly reduced this trap and new electron traps with activation energies of 0.8 and 0.9 eV are detected. These new traps are suggested to be due to individual nitrogen-related defect complexes, produced by the thermal annihilation of the N–N bond. After a high temperature treatment of the GaAsN growth melt with 0.1 wt.% Er, nitrogen is found to be removed from the layer and no electron trap was detected.

Deep levels in GaAs ultrashallow sidewall pin junctions measured by photocapacitance method

Photocapacitance measurements have been made on GaAs ultra-shallow sidewall p+i n+ junctions, fabricated by low-temperature area-selective re-growth molecular layer epitaxy. Emission spectra were obtained of the photocapacitance, affected by the deep levels at the re-growth interface region. From the photocapacitance results, it is shown that the defect levels and their density differed, depending on the device sidewall mesa orientations, with the magnitude of deep level density increasing in the order of: normal mesa < 45°-inclined configuration < reverse mesa orientation. When photocapacitance results of sidewall p+i n+ junctions were compared with current-voltage characteristics of sidewall p+n+ tunnel junctions, a good agreement was found. It is considered that the deep levels at the re-growth interface are a major factor controlling excess current in sidewall tunnel junctions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoresponse of (In,Ga)N–GaN Multiple-Quantum-Well Structures in the Visible and UVA Ranges

Characterization and analysis of photoresponse in p-n diodes with embedded (In,Ga)N-GaN multiple-quantum-well (MQW) structures are reported. Their dependence on the number of wells and In composition are considered. The influence of device structure on electric fields in the active region and on device responsivity has also been studied. Theoretical considerations as well as photocapacitance and photocurrent measurements show that the position of quantum wells (QWs), either in the quasi-neutral region or in the space charge region, is a critical factor in the collection efficiency. Hence, device photoresponse is not proportional to the number of QWs in photovoltaic mode. Present p-MQW-n devices show a promising performance as UVA and visible photodetectors, with detectivities, D/sup */, higher than 1.2/spl times/10/sup 12/ cm/spl middot/Hz/sup 1/2//spl middot/W/sup -1/ and rejection ratios higher than 10/sup 3/.

Transport Properties and Conduction Band Offset of n-ZnO/n-6H-SiC Heterostructures

ABSTRACTThe conduction band offset of n-ZnO/n-6H-SiC heterostructures fabricated by rf-sputtered ZnO on commercial n-type 6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and deep level transient spectroscopy measurements showed the conduction band offsets to be 1.25 eV, 1.1 eV, and 1.22 eV, respectively.